Borrelia burgdorferi, the etiologic agent of Lyme borreliosis, is the leading arthropod-borne infection in the United States with over 300,000 cases diagnosed per year. In endemic areas, untreated B. burgdorferi results in significant morbidity, where it can manifest as cardiac, neurologic, or musculoskeletal pathology. Despite its minimalist genome, B. burgdorferi is adept at dramatically modifying its gene regulatory network as it moves from the tick vector to mammalian hosts. Several important borrelial virulence determinants, including ospC and dbpA, are coordinately regulated with many other genes. One such gene, designated bba33, encodes a lipoprotein of unknown function. In an attempt to identify a functional role for BBA33, we employed a random peptide phage display screen and found that peptides containing collagen type VI sequences were recognized by BBA33. Subsequent studies confirmed that BBA33 recognized collagens type VI and IV in a dose-dependent manner. In addition, we found that BBA33 bound to the collagenous domain of the C1q complement protein. When we deleted bba33 from B. burgdorferi, the cells were essentially non-infectious based on several metrics, including imaging, cultivation, and quantitative PCR. Given the abundance of collagenous targets in known sites of borrelial colonization (e.g., dermis, lymph nodes, joint tissue), it is plausible that the loss of collagen binding might render . burgdorferi non-infectious. This, coupled with the inability to engage complement C1q, may result in enhanced innate clearance. Consistent with this contention, BBA33 inhibited C1q-dependent complement activation, suggesting that BBA33::C1q interactions protect B. burgdorferi from complement dependent clearance. Taken together, our preliminary data strongly support that BBA33::collagen/C1q interactions are required for the full pathogenic potential of B. burgdorferi. To test this assertion, we propose the following Specific Aims: (1) Characterize BBA33-host protein interactions. We have shown qualitatively that BBA33 binds type VI and type IV collagens, as well as C1q. Here we will obtain quantitative data on these interactions and engineer synthetic collagen-like domains to identify the BBA33 binding site and test whether they can inhibit binding. This information will be used to define the BBA33::collagen/C1q interaction at the molecular level; and (2) Determine the role of BBA33-mediated interactions in borrelial pathogenesis. Our results show that B. burgdorferi bba33 deletions are cleared rapidly, implying that BBA33 is required for the initial host tissue adherence and/or resistance to innate immunity, via structural collagens and C1q, respectively. Given that the clearance of serum sensitive Borrelia spp. requires C1q, and that exported BBA33 is missing from these same strains, it is possible that BBA33 mediates this response. In this regard, we will assess whether surface exposed BBA33 from B. burgdorferi prevents complement killing via C1q recognition. In addition we will determine the role that C1q plays in borrelial pathogenesis by infecting C1q knockout mice and evaluating the spatial and temporal infectivity pattern via in vivo imaging.